1. Field of the Invention
This invention relates to a sheet conveying apparatus for conveying sheet materials which is applied to an image forming apparatus such as an electrophotographic apparatus, an electrostatic recording apparatus or a laser beam printer.
2. Related Background Art
As image forming apparatus of this kind, there are, for example, electrophotographic apparatuses, electrostatic recording apparatuses, laser beam printers, etc. As a first example of the prior art, an image forming apparatus using an electrophotographic process will hereinafter be described with reference to FIGS. 22 and 23 of the accompanying drawings.
In FIGS. 22 and 23, the reference numeral 201 designates an image forming apparatus, and the reference numeral 202 denotes a feeding cassette containing sheet materials S therein. The feeding cassette 202 has the sheet materials S stacked therein, and is mounted on the image forming apparatus 201 from the direction of arrow A.
A feeding roller 203 is adapted to be rotated about the center of rotation 203a to thereby feed the sheet materials S into the image forming apparatus 201. A conveying roller 204 is rotatably installed in the image forming apparatus 201. A pinch roller 205 is rotatably installed in the image forming apparatus 201. This pinch roller 205 is pressed by pressing means (not shown) in order to form a nip with the conveying roller 204.
The reference numerals 206, 220 and 210 respectively denote a drive input gear, an idler gear, and a conveying roller driving gear. These gears are all rotatably installed in the image forming apparatus 201 and are connected together so as to be capable of transmitting drive. The conveying roller driving gear 210 is fixed in the direction of rotation with the conveying roller 204, and by the conveying roller driving gear 210 being rotated, the conveying roller 204 can be driven.
When the drive input gear 206 is rotated in the direction of arrow E by a main motor (not shown), the conveying roller 204 is rotated in the direction of arrow H through the idler gear 220 and the conveying roller driving gear 210. By this operation, the conveying roller 204 and the pinch roller 205 function to feed the sheet material S fed by the feeding roller 203 into the image forming apparatus 201.
Register rollers 211 convey the sheet material S at predetermined timing, and a process cartridge 212 contains a photosensitive drum 212a, a primary charger, a developing device, a cleaning device (not shown), etc. therein. The reference numeral 213 designates a transfer roller, and the reference numeral 214 denotes a fixating device containing a fixating roller 214a, a pressing roller 214b, etc. therein. The references numeral 215, 216 and 217 respectively designate first discharge rollers, second discharge rollers, and a discharge tray on which printed sheet materials S are to be stacked. A scanner unit 218 scans a laser beam L, and a mirror 219 directs the laser beam L to the photosensitive drum 212a. 
In the construction as described above, when the image forming apparatus 201 is instructed to effect printing by a host computer (not shown) connected thereto, the feeding roller 203 is rotated, whereby a sheet material S in the feeding cassette 202 is fed, and the fed sheet material S is conveyed into the image forming apparatus 201 by the conveying roller 214 and the pinch roller 215. The sheet material S is further directed to the process cartridge by the register rollers 211, and an image is printed on the surface of the sheet material S by a known printing process.
The sheet material S after transfer is further conveyed to the fixating device 214, where the image thereon is fixated, and then the sheet material S is discharged onto the discharge tray 217 by the first discharge rollers 215 and the second discharge rollers 216.
The prior art as described above, however, has suffered from the following problem.
In the first example of the prior art, there is a case where due to paper jam (hereinafter referred to as the xe2x80x9cjamxe2x80x9d) or other trouble, the image forming apparatus 201 is stopped with a sheet material nipped between the conveying roller 204 and the pinch roller 205 (the position of the sheet material S1 in FIG. 22).
In such case, in the construction of the prior art, a pressing force for forming the nip is acting between the conveying roller 204 and the pinch roller 205, and the conveying roller driving gear 210 is always connected to the main motor or other driving system, so that when the sheet material S1 is to be pulled out for the treatment of the jam, it has been necessary to rotate the main motor or other driving system.
Therefore, due to the load of the main motor or other driving system, a great force has become necessary to pull out the sheet material S1, and the treatability of the jam has been bad. Also, the sheet material S1 might be broken by the operation of pulling out the sheet material S1, and the broken sheet material S1 might remain in the image forming apparatus 201 to hinder the conveyance of the sheet materials S.
(Second Example of the Prior Art)
FIGS. 24 and 25 of the accompanying drawings show a second example of the prior art.
In FIGS. 24 and 25, portions common to those in the first example of the prior art are designated by reference numerals corresponding to those in the first example.
A feed roller 325 and a retard roller 326 form the nip therebetween, and the feed roller 325 is rotated in the direction of arrow V by driving means (not shown). The retard roller 326 is connected to a retard roller driving gear 328 through a torque limiter 327. The retard roller driving gear 328 is connected to an idler gear 320 so as to be capable of transmitting drive.
Accordingly, when the idler gear 320 is rotated in the direction of arrow F, the retard roller driving gear 328 is rotated in the direction of arrow R, whereby the drive force in the direction of arrow R is given to the retard roller 326 through the torque limiter 327. When predetermined or greater torque in the direction of arrow W is given to the retard roller 326, slip occurs in the interior of the torque limiter 327, so that the retard roller 326 is rotated in the direction of arrow W against the driving force of the retard roller driving gear 328 in the direction of arrow R.
By such construction, when a plurality of sheet materials S come into the nip portion between the feed roller 325 and the retard roller 326, only the uppermost sheet material S is conveyed.
During image formation, when the image forming apparatus 301 is instructed to effect printing by a host computer (not shown) connected thereto, a paper feeding roller 303 is rotated, to feed a sheet material S in a paper feeding cassette 302. When a plurality of sheet materials S are fed, only the uppermost sheet material S is conveyed into the image forming apparatus 301 by the feed roller 325 and the retard roller 326. An image is formed on the conveyed sheet material S, which is then discharged onto a discharge tray 317.
Again in the second example of the prior art, there is a case where when the image forming apparatus 301 is stopped due to jam or other trouble, a sheet material S1 is at the nip between the conveying roller 304 and the pinch roller 305 and at the nip between the feed roller 325 and the retard roller 326, as shown in FIGS. 24 and 25.
However, when the sheet material S1 is to be pulled out in the direction of arrow J when the jam has occurred, a pressing force for forming the nip is acting between the conveying roller 304 and the pinch roller 305, and between the feed roller 325 and the retard roller 326. Therefore, by the force with which the sheet material S1 is pulled out, drive force in the direction of arrow H acts on the conveying roller driving gear 310, so that a driving force in the direction of arrow F acts on the idler gear 320, and a driving force in the direction of arrow R acts on the retard roller driving gear 328. On the other hand, since the sheet material S1 is located at the nip between the feed roller 325 and the retard roller 326, a driving force in the direction of arrow W is given to the retard roller 326 by the force with which the sheet material S1 is pulled out.
Accordingly, the opposite driving forces in the direction of arrows R and W are given to the retard roller 326 by the operation of pulling out the sheet material S1 in the direction of arrow J, torque enough to cause slip in the interior of the torque limiter 327 becomes necessary. As the result, a great force becomes necessary to pull out the sheet material S1, which leads to the disadvantages that the treatability of the jam is bad, and the sheet material S1 is broken. Thus, the broken sheet material S remains in the image forming apparatus 301 and hinders the conveyance of the sheet materials S.
The present invention has been made in order to solve the above-noted problems peculiar to the first and second examples of the prior art, and has an object thereof to provide a sheet material conveying apparatus and an image forming apparatus, in which a load for pulling out a sheet material is reduced to thereby improve the treatability of jam and which is excellent in working property.
To achieve the above object, in the sheet material conveying apparatus provided with conveying means for conveying a sheet material while holding it by a driving force transmitted from a drive source, it is characterized by the drive transmission releasing mechanism for releasing the transmission of drive force to a drive source when the driving force is imparted from the conveying means. When the apparatus is stopped with the sheet material held by the conveying means and to be pulled out, the transmission of the drive to the drive force source is released by the drive transmission releasing mechanism. Therefore, the load for pulling out the sheet material is reduced.
The drive transmission releasing mechanism has a first gear provided on the drive source side and a second gear provided on the conveying means side, and when the first gear is rotated, it is connected to the second gear and the rotational force thereto, and when the second gear is rotated, it is not connected to the first gear not to transmit the rotational force thereto.
Further, the apparatus is provided with biasing means for biasing the first gear and the second gear toward each other, each of which are provided with projections having engagement surfaces engaged with each other and slide surfaces sliding respectively. With this feature, when the first gear is rotated in a predetermined direction, the engagement surfaces of the projections come into engagement with each other to connect the first gear with the second gear, and when the second gear is rotated in a predetermined direction. the slide surfaces of the projections slide relative to each other to move the second gear away from the first gear against the biasing force of the biasing means, and the first gear and the second gear are not connected together.
Also, in the sheet material conveying apparatus provided with a sheet cassette containing sheet materials therein, and conveying means for conveying the sheet materials fed from the sheet cassette by a driving force transmitted from a drive source while holding the sheet materials, it is possible to provide a drive transmitting mechanism for transmitting the driving force between the drive source and the conveying means only when the sheet cassette is set at a predetermined position. When the apparatus is stopped with a sheet material held by the conveying means and to be pulled out, if the sheet cassette is moved from the predetermined position, the driving force is not transmitted between the drive source and the conveying means. Therefore, the load for pulling out the sheet material is reduced.
Further, the present invention also has an object to obtain the above-described operational effect in a sheet feeding device in which a torque limiter is interposed between a retard roller opposed to a feed roller and a driving gear therefor.